Sloping/sinking floors?

House was built in 1968 - it's a split level. We are starting to experiencing some sloping on the top level. Seems like the lowest point is in the middle of the room. Noticing some wall cracking too but not sure if this is related. Do we need to call a structural engineer? How serious is this? We obviously want to do something to fix it but want to know is this a foundation issue we are looking at. We have been here for 8 years and plan to leave in next 6 -24 months. We had an inspection done and there were no concerns at the time when we moved in 8 years ago. It has gotten more visible in the last 2 years.